The present invention relates to an electrically operated power steering device which is fitted to the steering system of a road vehicle to provide supplementary steering power by means of an electric motor in general and, particularly to such a device with improved restoration characteristics on completion of a turn in either direction.
Electrically operated power steering devices generally provide supplementary steering power (assistance) by measuring the torsional torque applied when the steering wheel is operated, to drive an electric motor according to this torsional torque, the output torque of the electric motor being applied to the rack shaft in the case of a rack-pinion type steering gear. Such electrically operated systems require the motor to be driven only while steering, thus eliminating loss in of engine power incurred in maintaining a continuous flow of hydraulic fluid, such as required in hydraulically operated systems. Also, as the direction and magnitude of assistance is determined by the polarity and magnitude of motor current, control including corrections for vehicle speed can be made with relative ease. Further, although restoration characteristics tend to deteriorate in power steering systems due to increase in friction in the system, electrically operated systems have the advantage in that countermeasures can be taken more easily than with hydraulically operated systems.
With electrically operated systems however, the restoration characteristics and the "feel" of steering tend to suffer due to friction in the reduction gears, since the motor is connected to the rack shaft through reduction gears. Also, due to the increase in the amount of inertia of the motor, convergence in a hands-off condition at high speeds tends to be impaired.
An electrically operated power steering device intended for remedying such shortcomings has been described in, for example, the Japanese publication of examined patent application (JB, B1) No. 46-5168 (1971). In this prior art example, it has been proposed to drive the motor in the restoring direction subject to the output of a discriminator which detects such a condition where no input torque is applied on the steering wheel while the wheel is in any position other than neutral. However, this system works only when the input torque on the steering wheel is nil, and it also fails to provide adequate response, as the discriminator sends the output signal only after detecting that the above conditions have been met.